Because her initial purpose is to find the average hand span of the students in her class. If she asks the hand span of each student in the class then she can have many different answers and it anticipates variability in students' hand spans.
Ex: There are 5 students in her class, she asks 5 of them and their answer is:
0.5m 0.6m 0.7m 0.8m 0.9m
=> the average hand span of the students in her class = (0.5m + 0.6m + 0.7m + 0.8m + 0.9m) / 5 = 0.7 m
